First, the terms used in the following description are defined. Unless otherwise notified, the directions of an upright piano are those seen from the piano player's side; that is, “front”, “back”, “left”, and “right” are the front, back, left and right, respectively of the upright piano, from the perspective of the piano player. “Clockwise” and “counter-clockwise” are the clockwise and counter-clockwise, respectively, from the perspective of the piano player. The term a “rest state” means a “state where no force is applied by a piano player to a key”. It also means that “the front of a key closest to a piano player is located at the highest point of its travel” The term a “ready state” means a “state of an action during the time when the key is in the rest state”. The expression that a “pedal is in the activated state” means that “a piano player is pressing a pedal” The expression that a “pedal is in the released state” means “no force is applied to a pedal by a piano player”
A typical upright piano has 88 actions. Each action comprises a key, a whippen, a jack, a hammer butt, a hammer and a strong. The typical upright piano also has a shift pedal (see, Patent Document 1). The shift pedal is connected to a half-blow rail through a trap lever and a pedal rod. The half-blow rail is rotatablyrotatably supported by a hammer-rest rail. When the action is in the ready state, the hammer is abutted to the half-blow rail.
A hammer-butt flange is secured to an action rail. The hammer butt is pivotall rotatably y supported by the hammer-butt flange. The hammer butt supports the hammer. The hammer butt also supports a back stop through a back stop shank. The whippen is rotatably supported by the action rail. The jack is rotatably supported by the whippen. A protruding end of the jack is configured in such a manner that it can pushes upward a pushed-up portion of the hammer butt. A bridle wire and a back-check wire are projected from the whippen. A back check is secured to one end of the hack-check wire.
Each string is composed of one to three piano wires. An action of an upright piano has a bridle strap (see, Patent Document 2). The bridle strap is a unique part of upright pianos. Grand pianos have no bridle strap. The bridle strap connects the back stop and the bridle wire. More specifically, the rear end of the bridle strap is connected to the junction between the back stop and the back stop shank. The front end of the bridle strap is connected to the end of the bridle wire.
Unless otherwise specified, a “length of the bridle strap” or an “expansion of the bridle strap” refers to a “length of a segment of the bridle strap extending between a point where the bridle strap is connected to the back stop and a point where the bridle strap is connected to the bridle wire”. Unless otherwise specified, a “minimum length of the bridle strap” has a similar meaning.
When the action is in the ready state, a piano player applies a force to a key to depress the key. The whippen rotates upward, the jack pushes up the hammer butt, and the hammer rotates backward. Then, the jack escapes from under the hammer butt and the hammer is disconnected from the motion of the key. The hammer returns back due to the moment of inertia to hit the string. After hitting the string, the hammer rotates forward. The back check then captures the back stop and the hammer is stopped.
Next, the piano player releases the key. The whippen rotates downward, and the back check releases the back stop. The whippen further rotates downward, and the bridle wire rotates downward along with the whippen. The distance between the back stop and the bridle wire is increased, and the bridle strap pulls the hammer butt forward. The hammer butt rotates forward along with the hammer. The rotated hammer is abutted against the half-blow rail. The time interval from the point when the back check releases the back stop to the point when the hammer is abutted against the half-blow rail is denoted by T1. In addition, when the whippen falls, the jack slips under the hammer butt. The action is returned to its ready state.
When the piano player depresses the key, the protruding end of the pushing-up portion of the jack pushes up the pushed-up portion of the hammer butt. The protruding end of the pushing-up portion of the jack is made of wood or plastic. Thus, the protruding end of the pushing-up portion is hard. The pushed-up portion of the hammer butt is covered with a piece of non-woven fabric or leather. Thus, the pushed-up portion is soft. Such a portion of the pushed-up portion that is hit by the protruding end of the pushing-up portion is pressed and become thin with time. As a result, the pushed-up portion has an uneven surface or difference in level at a boundary between a portion hit by the protruding end of the pushing-up portion and a portion that is not affected by the protruding end of the pushing-up portion.
When the piano player presses a shift pedal, the shift pedal is shifted from the released state to the activate state. The shift pedal pushes up the pedal rod through the trap lever, and the pedal rod in turn pushes up the half-blow rail. The half-blow rail rotates backward. The hammer is pushed up by the half-blow rail and rotates backward to a closer position to the string. This means that the hammer travels a shorter distance when it rotates to hit the string while the piano player is depressing the shift pedal. With the shorter rotation distance of the hammer, a weaker sound is produced when the hammer hit the string. Thus, the piano player can change degrees of loudness or softness of a note by using the shift pedal.
In the following description, a “state a” means a “state where the action is in the ready state, and the shift pedal is in the released state.” A “state b” means a “state where the action is the ready state and the shift pedal is in the activated state”. In the state a, the piano player depresses the shift pedal. The hammer rotates backward along with the hammer butt, and the back stop moves away from the bridle wire. This situation corresponds to the state b. In the state b, a distance between the back stop and the bridle wire is denoted by D9. The distance D9 is a factor that determines the minimum length LB1 for a conventional bridle strap.
The length LB1 corresponds to a length with which the bridle strap has no slack in the state b. The length LB1 also corresponds to a length with which the bridle strap does not pull the hammer butt forward in the state b. When the length of the bridle strap is less than LB1, the following problems arise. In the state a, when the piano player depresses the shift pedal, the hammer and the hammer butt rotate backward. During this, the hammer butt pulls the bridle strap backward. The bridle strap pulls the whippen upward, and the whippen rotates upward. As a result, the tail of the key is lifted and the front of the key falls, both without the piano player's intent. In order to avoid this, the bridle strap has a length of at least LB1.